ALD is a well-known method of layer by layer film growth in which a substrate is alternately exposed to two or more gas phase reagents or precursors that are temporally or spatially separated from each other such that they cannot mix in the gas phase, but only react with each other on the surface of the substrate being coated by the film. Typically the precursors are separated by some type of purging or evacuation of a deposition chamber that is being held at reduced pressure. While ALD may be practiced in a regime where simple adsorption of a precursor is used to saturate the surface of a substrate, it is more typical that the precursors react with groups on the surface of the wafer to form a chemisorbed layer after each exposure. By-products of the chemisorption reactions are typically removed from the chamber during purging after each exposure step.
Further, ALD as practiced for industrial purposes is typically performed at moderate temperatures in the range of 100° C. to 400° C. Well known processes exist for deposition of metal oxide films such as Al2O3, HfO2, ZrO2, La2O3, Y2O3, etc., in addition to metal nitrides such as Ti3N4, Hf3N4, Ta3N5, etc. By introducing reducing reagents such as H2 or NH3 into the reaction sequence, usually in combination with a plasma as Plasma Enhanced ALD (PEALD), it is possible to deposit metallic/conducting films such as TiN, TaN, HfN, Ru, etc.
The major advantages of ALD and PEALD include the self-limited nature of the film growth which allows for excellent, in some cases near-perfect, film conformality over relatively high aspect ratio structures even at very small nanometer scale dimensions, and also allows for excellent thickness control and extremely low non-uniformity across large substrate areas. In addition, the temperature of ALD and PEALD depositions is typically lower than chemical vapor deposition (CVD) processes that deposit the same film with similar compositional purity, thus enabling a lower thermal budget for film deposition.
ALD has several limitations as well. For instance, it is typically slower than the corresponding CVD process because the deposition is done sequentially rather than in a continuous process. Thus, it is more economically feasible to use ALD to grow very thin films. In addition, ALD requires precursors that have a sufficient vapor pressure and thermal stability to be delivered in the gas phase. The substrate and deposition chamber need to be maintained at high enough temperature and low enough pressure to maintain the precursor and byproducts in the gas phase so that any excess precursor passes the substrate unreacted without building up residue from unwanted by-products.